Plasma Diagnostics and Modelling of Nanosecond Pulsed Surface Dielectric Barrier Discharge Actuators

During the past years, an increasing number of studies have been conducted on the use of electrical discharges for the stabilization of airflows (plasma flow control). Electrical gas discharges transfer energy and momentum to the gas through collisions of free electrons with atoms and molecules. Chemically active species such as ions, radicals and excited species are produced due to these collisions. The use of plasma actuators, notably surface dielectric barrier discharges (SDBD), for flow control applications has been largely investigated, and it has been demonstrated to effectively control the flow at low flow speed (below 30 m/s). Nowadays, the research in this area focuses on ways to improve the plasma actuators for flow speeds relevant to real flight conditions. One promising device for plasma flow control at high flow speed is the nanosecond pulsed surface dielectric barrier discharge. Nanosecond pulsed plasmas in general have also drawn attention from other fields, such as plasma assisted combustion, due to their ability to produce a large amount of active species and producing substantial overheating in a very short time. In this thesis an investigation of nanosecond pulsed SDBD is presented, with a focus on flow-control applications, but also on the production of active species, which are of great interest for plasma-assisted combustion and for other fields. The experimental characterization of the plasma created for different conditions, such as the operating pressure, the polarity and the amplitude of the applied voltage, is conducted. Important plasma parameters such as the gas temperature, the excited species produced, the reduced electric field or the electron density are either directly measured or inferred from emission spectroscopy using existing and novel diagnostic methods. The validity of the diagnostic methods is demonstrated using a numerical model of the plasma. The numerical modelling of the plasma also allows determining the influence of the plasma on the flow for several conditions. The experimentally studied conditions are simulated and compared with experimental results to show the strengths and limitations of the numerical model

Risk Exposure to Particles – including Legionella pneumophila – emitted during Showering with Water-Saving Showers

The increase in legionellosis incidence in the general population in recent years calls for a better characterization of the sources of infection, such as showering. Water-efficient shower systems that use water atomization technology may emit slightly more inhalable bacteria-sized particles than traditional systems, which may increase the risk of users inhaling contaminants associated with these water droplets. To evaluate the risk, the number and mass of inhalable water droplets emitted by twelve showerheads—eight using water-atomization technology and four using continuous-flow technology— were monitored in a shower stall. The water-atomizing showers tested not only had lower flow rates, but also larger spray angles, less nozzles, and larger nozzle diameters than those of the continuous-flow showerheads. A difference in the behavior of inhalable water droplets between the two technologies was observed, both unobstructed or in the presence of a mannequin. The evaporation of inhalable water droplets emitted by the water-atomization showers favored a homogenous distribution in the shower stall. In the presence of the mannequin, the number and mass of inhalable droplets increased for the continuous-flow showerheads and decreased for the water-atomization showerheads. The water-atomization showerheads emitted less inhalable water mass than the continuous-flow showerheads did per unit of time; however, they generally emitted a slightly higher number of inhalable droplets—only one model performed as well as the continuous-flow showerheads in this regard. To specifically assess the aerosolisation rate of bacteria, in particular of the opportunistic water pathogen Legionella pneumophila, during showering controlled experiments were run with one atomization showerhead and one continuous-flow, first inside a glove box, second inside a shower stall. The bioaerosols were sampled with a Coriolis® air sampler and the total number of viable (cultivable and noncultivable) bacteria was determined by flow cytometry and culture. We found that the rate of viable and cultivable Legionella aerosolized from the water jet was similar between the two showerheads: the viable fraction represents 0.02% of the overall bacteria present in water, while the cultivable fraction corresponds to only 0.0005%. The two showerhead models emitted a similar ratio of airborne Legionella viable and cultivable per volume of water used. Similar results were obtained with naturally contaminated hoses tested in shower stall. Therefore, the risk of exposure to Legionella is not expected to increase significantly with the new generation of water-efficient showerheads

Electron temperature measurements using N2 and Argon transitions in an ATP fast pulsed DBD

Dielectric Barrier Discharge in air at atmospheric pressur

Risk Exposure during Showering and Water-Saving Showers

Eco-friendly showers aim to lower energy and water consumption by generating smaller water droplets than those produced by traditional systems. To evaluate the risk of users inhaling the contaminants associated with such water droplets—namely, chemical components or opportunistic bacterial pathogens such as Legionella—we modeled the behavior of water droplets aerosolized by water-atomization technology at a flow rate of 2.2 L/min and compared the results obtained using this model with those determined experimentally in a typical shower stall. Additionally, we monitored the number and mass of inhalable water droplets emitted by twelve showerheads—eight using water-atomization technology and four using continuous-flow technology—which have distinct characteristics in terms of water flow rate, water pressure, spray angle, and number of and diameter of nozzles. The water-atomizing showers tested not only had lower flow rates, but also larger spray angles, less nozzles, and larger nozzle diameters than those of the continuous-flow showerheads. We observed a difference in the behavior of inhalable water droplets between the two technologies, both unobstructed and with the presence of a mannequin. The evaporation of inhalable water droplets emitted by the water-atomization showers favored a homogenous distribution in the shower stall. In the presence of the mannequin, the number and mass of inhalable droplets increased for the continuous-flow showerheads and decreased for the water-atomization showerheads. The water-atomization showerheads emitted less inhalable water mass than the continuous-flow showerheads did per unit of time; however, they generally emitted a slightly higher number of inhalable droplets (1.6 times more), including those large enough to carry a bacterium each—only one model performed as well as the continuous-flow showerheads in this regard. Further experiments are needed to assess whether this slight increase in the number of inhalable water droplets increases the biological risk

Investigation of Nanosecond Pulse Dielectric Barrier Discharges in Still Air and in Transonic Flow by Optical Methods

In the present study the interaction of nanosecond pulsed dielectric barrier discharge (ns-DBD) actuators with aerodynamic flow up to transonic velocities was investigated. The primary focus was on the influence of the flow on the discharge and the effects of the discharge itself. In addition, the influence of the ns-DBD on a shock-wave was studied. The aim was to improve the understanding of the plasma-flow interaction, a topic that is not yet fully understood, in particular for ns-DBD. The actuator was integrated in two different models, a NACA 3506 compressor blade profile and a bump geometry at the bottom of the wind tunnel. The effect of the rapid energy deposition close to the discharge was examined with the phase-locked schlieren visualisation technique. Images of the plasma acquired with short exposure times revealed information on the discharge evolution. The results show a significant effect of the flow on the discharge characteristics, in particular due to the drop of static pressure. On the other hand, no significant effect of the ns-DBD on the flow was observed due to unfavourable flow conditions, which underlines the importance of the actuator’s placement

Aero Engine Flow Control by Surface Discharge Plasma

Experimental investigations and numerical modeling with the objective to study the effects that surface discharge plasmas can provoke on aerodynamic flow situations, including high speed flows present in aero-engines, are undertaken at EPFL. The actuator’s potential to influence the flow under realistic flight and engine operating conditions, largely depends on its geometric configuration and the power supply. Initially, a long-life (more than 12 hours) surface discharge actuator was designed at EPFL and proven to survive transonic flow speeds on typical compressor blades. This was the first time that such a long-term, continuously operating actuator was applied successfully. However, with AC voltage applied to the electrodes, the effects on the shock waves did not appear to be significant [1, 2]. Recent experiments conducted at EPFL focused on fast rise voltage pulse driven dielectric barrier discharge (DBD) actuators and the development of a power supply that could provide the required voltage pulses. A generic DBD actuator with two electrodes in asymmetric configuration, separated by a dielectric was chosen. High speed imaging was applied to investigate the spatial and temporal discharge development. In order to study the influence of voltage and current on the plasma, several power supply configurations were examined by varying voltage rise and fall time, maximum voltage, peak current and pulse width. Two discharge periods were observed on the positive and the negative edge of the voltage pulse. It was observed that the characteristics of the plasma differ considerably between these two periods. Furthermore, the light emission intensity and the propagation speed of the plasma along the surface of the dielectric increased with the maximum voltage and peak current. A similar effect was observed when the voltage rise time was decreased. In a next step, a technique similar to synthetic schlieren was applied to examine the interaction of the DBD actuator with air under atmospheric pressure. A micro shock wave that propagates from the electrode edge normal to the surface was observed, which agrees well with numerical results gained during a parallel investigation. In addition, the impact on the flow structure of maximum voltage and frequency is currently examined. First results suggest that the intensity of the micro shock wave increases with the voltage whereas the frequency primarily provokes electrode heating. With the objective to investigate the modelling of the fast rise pulse driven DBD discharge, a numerical tool was developed that applies emission spectroscopy to determine the electron energy distribution function (EEDF) by comparing the relative intensity of bands, as described by several authors [3-5]. The EEDF is then used to infer populations present in the gas through electron impact ionization, excitation or dissociation cross sections data [6]. Contrary to previous studies, this approach employs two naturally occurring transitions in atmospheric plasmas in conjunction with recent data on collisional quenching [7], which makes the method more reliable and does not require the adjunction of other species such as Helium. References: 1. Pavon S., Dorier J.L., Hollenstein C., Ott P., Leyland P., “Effects of high-speed airflows on a surface dielectric barrier discharge”, J. Phys. D: Appl. Phys. 40. No 6.2 1733-1741, 2007. 2. Pavon S., Sublet A., Dorier J.L., Hollenstein C., Ott P., Leyland P., “Long Lifetime system for the generation of Surface Plasmas”, International Patent no: P1883PC00/13-71, PCT/IB2009/050489, 2008. 3. Bibinov N. K., Kokh D.B., Kolokolov N.B., Kostenko V.A., Meyer D., Vinogradov I.P., Wiesemann K., “A comparative study of the electron dis-tribution function in the positive columns in N2 and N2/ He dc glow dis-charges by optical spectroscopy and probes”, Plasma Sources Science and Technology, 298-309,1998. 4. Behringer K., Fantz U., “Spectroscopic diagnostics of glow discharge plasmas with non-maxwellian electron energy distributions”, Journal of Applied Physics D, 2128-2135, 1994. 5. Isola L.M., Gómez B.J., Guerra V., “Determination of the electron tem-perature and density in the negative glow of a nitrogen pulsed discharge using optical emission spectroscopy”, Journal of Applied Physics D, 015202, 2010. 6. Itikawa Y., “Cross sections for electron collisions with nitrogen mole-cules”, Journal of Physical and Chemical Reference Data, 31-53, 2006. 7. Dilecce G., Ambrico P.F., Benedictis S., “On the collision quenching of N2plus by N2 and O2 and its influence on the measurement of E over N by intensity ratio of nitrogen spectral bands”, Journal of Applied Physics D, 195201, 2010

OES Characterization of Steamers in a Nanosecond Pulsed SBDB Using N2 and Ar Transitions

The characterization of non-thermal homogeneous plasmas is possible using optical emission spectroscopy (OES), notably by estimating the reduced electric field. This method was applied to characterize streamers generated by a nanosecond pulsed surface dielectric barrier discharge (SDBD) operated in quiescent air at atmospheric pressure and also at 0.5 atm. The average reduced electric field associated with the surface streamers was determined using four different sets of transitions occurring in air plasmas, the first negative system (FNS) of N2 + , the first positive system (FPS) and second positive system (SPS) of N2 and argon transitions 2px − 1sy. The analysis of the results allowed to critically assess the validity of the estimated reduced electric field for the present conditions. It is shown experimentally that the inhomogeneous nature of the streamer head influences significantly the estimation of the reduced electric field. Moreover, the estimated reduced electric field is not sufficient to characterize the processes taking place in the streamer head, due to the steep variation of both the reduced electric field E/N and the electron density ne in space and time. To overcome this limitation, a new method is proposed to take into account the spatial structure of a streamer head. The applicability of the new method is demonstrated for these experimental conditions and shows a very good agreement for the transitions tested

Interaction Between Nanosecond Pulse DBD Actuators and Transonic Flow

The application of nanosecond pulse-driven surface dielectric barrier discharge (DBD) actuators as control devices on airfoils and turbomachinery blades in transonic flow is investigated experimentally. Images acquired with a short-exposure ICCD camera document the spatial-temporal discharge development in both absence and presence of flow, as a means of examining the effect of the ow on the actuator. In order to visualize the interaction of the DBD actuator with the ow, Schlieren images were acquired. Furthermore, a power supply capable of generating the required voltage pulses was built and characterized. The impact of the pulse transition time on the discharge development was also investigated

Experimental Investigation of Pulsed Dielectric Barrier Discharge Actuators in Sub- and Transonic Flow

Experiments were conducted in order to investigate the ability of nanosecond pulsed dielectric barrier discharge (ns-DBD) actuators to control flow on airfoils in subsonic and transonic flow (up to Ma1 = 0.75, Re = 1.35 • 10^6). A NACA 0015 profile equipped with a leading edge mounted ns-DBD actuator was investigated up to Re = 2.3 • 10^5 (u1 = 24 m/s). Measurements of the surface pressure distribution clearly confirm the actuator’s potential to delay leading edge separation. In the following the impact on the control authority of different voltage pulse parameters, such as voltage amplitude, actuation frequency and rise/fall time of the pulse were investigated. The experiments in transonic flow were conducted on a NACA 3506 compressor blade profile. A ns-DBD actuator was placed at x/c = 0.33 where the foot of the shock-wave and boundary-layer separation was observed. Schlieren flow visualization showed the shock-wave boundary-layer interaction and was used to investigate the actuator’s effect on the shock position and shape. A high-speed camera allowed to acquire schlieren images at high acquisition rates and investigate as well the movement of the shock in the frequency domain. These results were verified with measurements of the static pressure at the side wall using unsteady pressure transducers